Laser self-mixing occurs if an external reflecting surface is arranged within the optical path of a laser so that an external cavity is obtained. Tuning of the external cavity results in a readjustment of the laser equilibrium conditions and thus to detectable changes in the laser output power. These changes, typically in the form of undulations or oscillations, are repetitive as a function of the displacement of the external reflecting surface over a distance of a half laser wavelength. The undulation frequency is proportional to the velocity of the external reflector. A measuring device based on laser self-mixing typically shows a high sensitivity and thus a high accuracy. This can be attributed to the fact that the reflected light re-entering the laser cavity determines the modulation frequency of the laser light and is thus amplified in the laser cavity. In this way, the laser acts as a phase-sensitive detector and amplifier. Accordingly, a high receiver sensitivity is obtained without additional means such as optical filters or complex devices such as interferometers. For example, a laser self-mixing device is known from U.S. Pat. No. 6,707,027 B2. This device makes use of the Doppler phase shift occurring if the reflecting surface has a component of movement along the optical path.
If, however, a laser self-mixing device is used to gauge the velocity or distance of an object having a randomly reflecting surface, the reflected signal strongly depends on the spatially variable reflectivity caused by wavelength-scale structures and/or variations of composition. This may lead to very low reflected signal levels which result in low or even vanishing measuring signals, e.g. on absorptive or black surfaces, but also on very reflective surfaces such as on mirrors, glass or shiny surfaces. This particularly applies to measuring geometries in which the specular beam is not reflected back into the laser cavity.
Another effect is what is called the speckle pattern which is caused by the wavelength-scale structure of the surface. The speckle pattern gives rise to strong random intensity variations which in turn also modulate the self-mixing signal amplitude, frequency and phase. Accordingly, the speckle pattern influences the measurement accuracy of the system with regard to parameter gauges using the undulation or oscillation signal.